Passively Q-switched microchip lasers (MCLs) have been demonstrated as a promising concept for realizing compact laser-sources for various applications. MCLs employing neodymium doped yttrium vanadate (Nd:YVO4) as a solid-state gain medium and passively Q-switched by a semiconductor saturable absorber mirror (SESAM) can generate pulses with durations in the 100 picosecond (ps) range with energies of several 100 nanojoules (nJ). One microchip laser suitable for use with this gain medium is described in detail in U.S. Pre-grant Publication No. 2011/0243158, the complete disclosure of which is hereby incorporated herein by reference.
Such MCLs have inherently single longitudinal mode operation, due to the resonator length being short enough that only one lasing mode is possible within the gain-bandwidth of the gain-medium, and exhibit nearly diffraction-limited beam quality. This makes these MCLs potentially suitable for applications including spectroscopy, frequency conversion, micromachining, light detection and ranging (LIDAR), and precision medical and dental operations. Where additional power is required, the output of an MCL can be optically amplified creating a MOPA system with an MCL as master oscillator. One optical amplifier suitable for this purpose is described in U.S. Pat. No. 7,256,931, incorporated herein by reference. This amplifier is a compact, multi-pass, grazing-incidence amplifier employing a thin, relatively short slab of Nd:YVO4, faced-pumped by a diode-laser array (diode-laser bar). The compact nature of the amplifier, in conjunction with the compact MCL provides for a correspondingly compact MOPA system.
In present such MOPA systems, the passively Q-switched MCL delivers a train of pulses at some PRF determined, inter alia, by properties of a saturable absorption element such as a semiconductor saturable absorption mirror (SESAM) providing the passive Q-switching, and optical pump power supplied to the solid-state gain-medium of the laser. The amplifier amplifies the pulse-train from the laser to provide a train of amplified pulses as the MOPA output.
In many applications for which such a MOPA is suitable, it can be an advantage to deliver individual pulses having an arbitrary temporal separation therebetween, bursts of pulses having the laser-PRP between pulses in the burst, or a train of pulses having a PRP significantly different from that of the passively Q-switched laser. In current such MOPA systems, this is typically accomplished by providing an electro-optical switch, such as an acousto-optic modulator or an electro-optic modulator inside the laser oscillator. Opening the switch on demand leads to the evolution of a giant-pulse. This switching scheme is so called “active Q-switching”. Another approach is to place the switch at the MOPA (amplifier) output of a passively Q-switched MOPA. The switch is electronically triggered to pass a pulse or pulses as required, rejecting those pulses not required. Such a switch is often whimsically referred to as a “pulse picker” by practitioners of the art.
A problem presented by active Q-switching is that a relatively long laser-resonator is required to accommodate the Q-switch leading to longer pulses. In the passive Q-switched, modulated output approach, the rejected pulses must be contained in some way. This is usually accomplished by directing the rejected pulses into an absorbing “beam dump”. This usually results in the generation of heat, which must be removed either by active or passive cooling means. Another disadvantage of this approach is that it is incompatible with generating pulses with arbitrary temporal separation. These problems could be avoided if a passively Q-switched laser were available that delivered pulses only on demand.